webadmin: Disabling Add Bricks if empty Cluster selected

In Create Volume dialog, the Add Bricks button will be
disabled if the selected Cluster is Empty or does not
have any hosts in UP state..

Change-Id: I1cdbdddd52fe29b8f652e7bf264bc70aa82b198e
Bug-Url: https://bugzilla.redhat.com/883774
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>